Generally, a printed circuit board is an electronic component electrically connecting various kinds of electronic components mounted thereon to each other in a board shape.
The printed circuit board is mainly divided into a rigid printed circuit board and a flexible printed circuit board according to a rigid or flexible material of a substrate, and recently, a rigid-flexible composite printed circuit board has been developed.
Initially, in the printed circuit board, a product having a relatively simple structure such as a structure in which a printed wiring is formed on a single side thereof was mainly used. However, in accordance with the gradual trend toward lightness, miniaturization, and multifunction of the electronic product, also in the flexible circuit board, wiring density has been increased and the structure thereof has been complicated and developed to the multi-layered product.
As the printed circuit board, there are various kinds such as a single layer, double side, multilayer, and the like, according to a layer of the circuit pattern of the wiring structure, and a suitable printed circuit board is designed and manufactured according to a structure and functions of an electronic device to thereby be applied to the product.
Particularly, the flexible printed circuit board may enable miniaturization and lightness of the electronic product and have excellent flexibility and softness, thereby freely connecting two circuits or components to each other that are not adjacent to each other while performing a role of the printed circuit board. Therefore, the flexible printed circuit board has been widely used in military equipment, medical equipment, general industrial machine, or the like, as well as electronic devices such as a mobile phone, an MP3, a camcorder, a printer, a display, or the like. Particularly, a product requiring flexural property of the circuit board such as the mobile phone, the camcorder, a notebook, the display, or the like, has increased, a demand for the flexible printed circuit board has increased.
A general method for manufacturing a double-sided printed circuit board among the printed circuit boards, for example, a method for manufacturing a double-sided flexible printed circuit board will be described as follows. After a double-sided copper clad laminate (CCL) film in which thin-film copper is laminated on both sides of an insulation film such as a polyimide film or a polyester film is prepared, via-holes are formed at a predetermined position of the CCL film using a drill, or the like, in order to electrically connect portions to each other at which a circuit pattern of the copper layer is formed, and then a plating process is performed on the via-hole, such that the copper layers are electrically connected to each other. Next, the double-sided flexible printed circuit board may be manufactured by a method of using a photosensitive film in both of the copper layers of the CCL film or applying liquid thereto to process each of the copper layers to a predetermined circuit pattern through exposing, developing, etching, delaminating processes.
In the manufacturing method according to the related art, a fine pattern may be formed, but a manufacturing process is complicated, raw material loss is significant, environmental pollution is generated. Recently, as a printed electronics technology is developed, a method for manufacturing a printed circuit board using a printing method has been developed, but in the current printing technology, there is a limitation in a width of the printed wiring.
Meanwhile, a method for manufacturing a double-sided flexible printed circuit board simultaneously using the etching method and the printing method is closed in Japanese Patent Application Laid-Open No. 1994-224528.
In the above manufacturing method, a through-hole is formed at a portion at which inner and outer surfaces of a film substrate are electrically connected to each other. At the same time, a metal foil is deposited on one entire surface of the film substrate, this metal foil is removed by the etching process so as to have a predetermined pattern to form a wiring conductor part, and closing plate part closing a portion of the through-hole is formed. In the other surface of the film substrate, a conductive paste is filled in the through-hole with printed wiring conductor part formed by depositing the conductive paste thereon using the printing method, and the wiring conductor part formed by the etching process using the conductive paste and the printed wiring conductor part formed by the printing method are electrically connected to each other, thereby manufacturing the double-sided flexible printed circuit board.
However, in the method, the printed wiring should be formed of the conductive paste by the printing method and at the same time the through-hole should be filled with the conductive paste, but the printing method for forming the printed wiring conductor part using a conductive paste filled in the through-hole to form a bump is extremely limited, and on the contrary, it is difficult to fill a conductive paste capable of easily forming the printed wiring in the through-hole to form the bump. In addition, in the flexible printed circuit board manufactured by the method, it is highly likely that a connection part formed at the through-hole is contracted or a crack is generated even when the connection part is exposed to heat or physical impact to thereby be disconnected, and in view of process, a process of forming a separate closing plate part in order to prevent the conductive paste filled in the through-hole from being leaked should be added. Therefore, the method is not industrially used. Further, since a conductive paste layer does not have sufficient adhesive force with the substrate, an interface between the printed circuit formed by the conductive paste and a connection conductor part forming the bump of the via-hole is often separated or detached from each other. Therefore, the method is not substantially practicable.